1. Field of the Invention
The present invention relates to a vibration actuator and to a lens barrel and camera equipped therewith.
2. Description of the Related Art
Heretofore, vibration actuators have been known in which expansion and contraction of an electromechanical conversion element is used to generate progressive vibration waves (hereinafter referred to as progressive waves) at a driving surface of a vibrating body, elliptical movements are produced at the driving surface by the progressive waves, and a relative motion member that pressingly touches against wave peaks of the elliptical movements is driven.
In recent years, reductions in size of these vibration actuators have been demanded. Among vibration wave actuators with configurations in which the resilient member has a substantially annular form and the relative motion member is rotatingly driven or the like, actuators with larger diametric direction widths produce greater driving force. When these vibration actuators are reduced in size, it is necessary to increase the diametric direction width of the resilient member in order to obtain driving force. Therefore, as the resilient member is reduced in size, there is a tendency for a ratio between the external diameter and the internal diameter of the resilient member to increase.
When the ratio between the external diameter and internal diameter of the resilient member increases, a difference between vibration amplitudes of the progressive waves at the outer periphery of the driving surface and vibration amplitudes of the progressive waves at the inner periphery becomes larger. Consequently, a problem arises in that driving of the relative motion member becomes unstable, losses of vibration occur, and driving efficiency of a vibration wave motor falls.
As a measure to reduce the difference between the vibration amplitudes of progressive waves at the outer periphery and the vibration amplitudes at the inner periphery, Patent Reference 1 discloses a technique of forming indentation portions (grooves) in the face of the relative motion member that touches against the vibrating body (a driving surface). The indentation portions do not reach as far as the outer periphery end of the vibrating body. Thus, thickness at the outer periphery of the vibrating body and thickness at the inner periphery are altered, and stiffnesses thereof with respect to bending displacements are altered (for example, see Japanese Unexamined Patent Publication No. H3-273874).